1. Field of the Invention
This invention relates generally to an error correcting and concealing system, and more particularly is directed to an error correcting and concealing system which is applied to a digital signal processing apparatus, such as, a digital video tape recorder.
2. Description of the Prior Art
In recording and reproducing a digital video signal by a rotary head type video tape recorder (VTR), random errors may result from head noise, tape noise or amplifier noise or a burst error may be caused by a signal dropout. It is a recognized advantage of digital signal processing that erroneous data can be mathematically corrected by the inclusion in the recorded or transmitted data of redundant bits. For example, a well known scheme for correcting digital data involves dividing the latter into blocks, each of which is recorded or transmitted along with a parity obtained by modulo 2 addition for each block so that, upon reproduction or reception, an error in any such block can be corrected on the basis of the respective parity. However, the addition to the recorded information data of the redundant bits representing the parity for the purpose of protecting the information data from errors necessarily increases the recording bit rate which is limited by the necessity of minimizing the consumption of tape. Therefore, even if the code arrangement of the digital video signal is designed to be capable of error correction, the extent of the error may sometimes exceed the error correcting ability which is limited by the acceptable redundancy.
It has further been proposed to conceal an error in a transmitted or recorded video signal so that such error will not be noticed in the displayed picture.
One error concealing method that has already been proposed involves interpolation of the erroneous data with data of the immediately preceding line of the same field, and such method relies on the strong correlation of a television picture in the vertical direction. Another conventional error concealing method involves replacement of the erroneous data with a mean value of data from the lines immediately preceding and following the line containing the error.
The above error concealing methods both obtain a signal for interpolation or substitution for the erroneous data from the data of the same field. Incidentally, since the television picture is formed by interlaced scanning, it will be appreciated that adjacent lines in the same field are spaced apart by a distance that is twice the distance between adjacent lines in the pictorial representation of the complete frame made up of two interlaced fields. Therefore, the data in immediately adjacent lines, respectively, of such pictorial representation of the complete frame and which occur in contiguous fields of the video signal have an even higher correlation therebetween.
Accordingly, it has been proposed by the assignee of this application to effect error concealment by replacing error-containing data in a line of one field with corresponding data in the line of the next previous field which, in the pictorial representation of the complete frame, is positioned immediately adjacent the error-containing line so that the data used for concealing an error will bear a closer resemblance to the original or correct data which it replaces.
If the error concealment technique is relied upon exclusively, picture degradation becomes a problem, particularly after multiple generations of dubbing, unless the rate or error occurrence is very low.